Gears are used in a number of different mechanisms. For instance, gears are often used in cars to translate power provided by an engine into movement via rotating wheels. Gears may also be used in rotorcraft to drive the rotation of rotors. Examples of gears that may be used in different vehicles or devices may be appreciated from U.S. Pat. Nos. 3,996,816, 5,083,474, 5,315,790, 6,128,969, 7,581,695, and U.S. Patent Application Publication Nos. 2002/0134184, 2004/0088861 and 2010/0132493.
Often, gears are shaped or configured to improve the impingement, bearing or shaft losses that may occur from intermeshing gears rotating. For instance, losses of energy that result from the friction that occurs when gears mesh with (i.e., contact) one another during rotational movement may be lessened by some structural changes to the gear teeth. These changes to the gear teeth are made along the entire profile (cross-sectional shape) of the teeth. Therefore, gear teeth typically all have a uniform shape throughout the entire width of the gear. U.S. Pat. No. 3,996,816, for instance, illustrates changes in gear teeth shape that may be made to improve energy losses that may result from meshing. Gear tooth modifications that reduce these meshing losses do not impact another important gear loss, windage loss.
Gear windage losses refer to the power losses that are a result of aerodynamic forces (pressure and viscous) acting against the rotation of high-speed gears. Windage losses are a source of significant heating and fuel consumption in rotorcraft and other VTOL systems. The weight and packaging constraints inherent in these systems require the gearing components to be both lightweight and heavily loaded. Attendant to this, the gears are required to operate at high rotational speeds where windage losses become significant with respect to other gearbox losses (meshing, sliding, and lubrication losses).
Windage loss is a phenomenon that occurs when gears rotate. Gears that rotate at relatively low speeds usually experience a negligible amount of windage loss. As a result, those of ordinary skill in the art have essentially no motivation to design gears that are used in low speed rotational systems to address windage loss. For example, gears used in cars or trains are generally not designed with addressing windage loss as a design objective.
Windage loss may became a major source of inefficiency in high speed rotation systems that utilize gears rotating at high speeds, such as a speed that corresponds to tip speeds greater than a Mach number of 0.2, which is about 68 m/s. Generally, those of ordinary skill in the art are unaware of the cause of windage loss. It has previously been determined, however, that the use of shrouds that cover gears may help reduce windage loss. For example, U.S. Pat. No. 5,048,370 discloses the use of such shrouds. The use of shrouds and baffle configurations for such shrouding may reduce windage loss and may also reduce the consumption rate for lubricating oil used in conjunction with gears in a drive system. However, the physics explaining why the shrouding and baffle configurations have such an impact have not been the subject of much research and are not well understood.
The shrouding and baffling are features that must be positioned adjacent to the gears. Shrouds and baffles do not rotate. They need to be reasonably heavy for structural integrity, and they need to completely surround the gears with minimum oil drainage holes or other leakage paths (or else they will not reduce windage loss efficiently). Accordingly, the use of shrouding and baffling for windage loss has practical limitations.
A new gear is needed that provides a significant reduction in windage loss relative to conventional gear designs used in high speed rotational systems. Such a gear design is preferably shaped and configured to permit gears to be easily manufactured using conventional manufacturing techniques. Such gear designs are also preferably useable in conjunction with shrouding or baffle configurations to further enhance the reductions in windage loss that may be obtained by the new gear designs.